Add ingestion of VMCATCHER_EVENT_AD_MPURI

As per https://github.com/hepix-virtualisation/vmcatcher/pull/8 , vmcatcher now provides also the image marketplace uri under the variable VMCATCHER_EVENT_AD_MPURI . This patch is to add this information into the OpenNebula image template, for two reason:
 1. Identifying the marketplace source of an image
 2. Have the marketplace information in the middleware to fill the BDII
Merge pull request #10 from spinto/patch-1

Add ingestion of VMCATCHER_EVENT_AD_MPURI